Gas cushion susceptor system

ABSTRACT

An apparatus and method to position a wafer onto a wafer holder and to maintain a uniform wafer temperature is disclosed. The wafer holder or susceptor comprises a recess or pocket whose surface includes a grid containing a plurality of grid grooves that separate protrusions. A plurality of gas passages is provided in the susceptor to enable an upward flow of gas toward the bottom surface of the substrate. During drop-off of the substrate, a cushion gas flow is provided to substantially slow the rate of descent of the substrate onto the susceptor and to gradually heat the substrate before it makes contact with the susceptor. Optionally, a trickle gas flow may be provided through the aforementioned passages during processing of the substrate to prevent deposition of reactant gases onto the bottom surface of the substrate. A liftoff gas flow may then be provided through the passages to help lift the substrate off of the susceptor after processing is completed and thus aid in removing the substrate from the process chamber. These features help to achieve temperature uniformity and thus quality of deposited films onto the substrate.

FIELD OF THE INVENTION

[0001] The present invention relates generally to structures forsupporting semiconductor substrates in process chambers, and, moreparticularly, to susceptors for radiantly heated semiconductor reactors.

BACKGROUND

[0002] Semiconductor fabrication processes are typically conducted withthe substrate supported within a chamber under controlled conditions.For many processes, semiconductor substrates (e.g., silicon wafers) areheated inside the process chamber. For example, substrates can be heatedby direct physical contact with a heated wafer holder and/or byradiation from a radiant heating source. “Susceptors,” for example, arewafer supports that absorb radiant heat and transmit absorbed heat tothe substrate. Unless otherwise indicated, the terms “substrate” and“wafer” are used interchangeably herein.

[0003] In a typical process, a reactant gas is passed over the heatedwafer, causing the chemical vapor deposition (CVD) of a thin layer ofreactant material on the wafer. Through sequential processing, multiplelayers are made into integrated circuits. Other exemplary processesinclude sputter deposition, photolithography, dry etching, plasmaprocessing, and high temperature annealing. Many of these processesrequire high temperatures and can be performed in the same or similarreaction chambers.

[0004] Various process parameters must be controlled carefully to ensurehigh quality deposited films. One critical parameter is the temperatureof the wafer during processing. During CVD, for example, there is acharacteristic temperature range within which the process gases reactmost efficiently for depositing a thin film onto the wafer. Temperaturecontrol is especially critical at temperatures below the mass transportregime, such as about 500° C. to 900° C. for silicon CVD using silane.In this kinetic regime, if the temperature is not uniform across thesurface of the wafer, the deposited film thickness will be uneven.

[0005] In recent years, single-wafer processing of large diameter wafershas become more widely used for a variety of reasons, including the needfor greater precision in process control than can be achieved withbatch-processing. Typical wafers are made of silicon, most commonly witha diameter of about 150 mm (about 6 inches) or of about 200 mm (about 8inches) and with a thickness of about 0.725 mm. Recently, larger siliconwafers with a diameter of about 300 mm (about 12 inches) and a thicknessof about 0.775 mm have been utilized because they exploit the benefitsof single-wafer processing even more efficiently. Even larger wafers areexpected in the future. A typical single-wafer susceptor includes apocket or recess within which the wafer rests during processing. In manycases, the recess is shaped to receive the wafer very closely.

[0006] There are a variety of quality control problems associated withhandling of substrates. These problems include substrate slide, stick,and curl. These problems primarily occur during placement and subsequentremoval of substrates in high temperature process chambers, particularlysingle-wafer chambers.

[0007] Substrate “slide” or “skate” occurs during drop off when acushion of gas in the susceptor recess or pocket is unable to escapefast enough to allow the substrate to fall immediately onto thesusceptor. The substrate floats momentarily above the susceptor as thegas slowly escapes, and it tends to drift off-center. Thus, thesubstrate may not rest in the center of the pocket as normally intended,and uneven heating of the substrate can result. Such drifting of thesubstrate to the edge of a susceptor pocket causes local thermalanomalies where the substrate is in contact with the pocket edge andresults in poor thickness uniformity, poor resistivity uniformity, andcrystallographic slip, depending on the nature of the layer beingdeposited. Non-uniformities in temperature can similarly causenon-uniformities in etching, annealing, doping, oxidation, nitridation,and other fabrication processes.

[0008] During substrate pick-up, “stick” occurs when the substrateclings to the underlying support because gas is slow to flow into thesmall space between the wafer and the surface of the substrate supportpocket. This creates a vacuum effect between the substrate and thesubstrate support as the substrate is lifted. Stick can contribute toparticle contamination due to scratching against the substrate supportand, in extreme cases, can cause lifting of the substrate holder on theorder of 1 to 2 mm.

[0009] Substrate “curl” is warping of the substrate caused by radial andaxial temperature gradients in the substrate. Severe curl can cause thesubstrate to contact the bottom side of a Bernoulli wand when a coldsubstrate is initially dropped onto a hot substrate support. Curl cansimilarly affect interaction with other robotic substrate handlingdevices. In the case of a Bernoulli wand, the top side of the substratecan scratch the Bernoulli wand, causing particulate contamination on thesubstrate. This significantly reduces yield. The design and function ofa Bernoulli wand are described in U.S. Pat. No. 5,997,588, the entiredisclosure of which is hereby incorporated by reference herein.

[0010]FIGS. 1A and 1B show a wafer 1 supported upon a conventionalsusceptor 100, wherein the susceptor 100 has a gridded support surfaceG. Referring initially to FIG. 1A, a portion of the wafer 1, close to aperipheral edge 2 thereof, is shown on the grid G. An upper surface ofthe grid G includes a plurality of projections 3 separated from oneanother in two dimensions by a plurality of grid grooves 101. Theseprojections 3 are recessed with respect to the upper surface of anannular shoulder 4 surrounding the grid. The top surface of the wafer 1rises slightly above the top surface of the shoulder 4, which helps tomaintain laminar gas flow over the wafer. An outer circumference 5 ofthe grid G is separated from an inner edge 6 of the shoulder 4 by anannular groove 7, which is generally semicircular in cross section. Thedepth of annular groove 7 into the susceptor 100 is about the same asthe depth of the grid grooves. The diameter of the inner edge 6 of theshoulder 4 is slightly larger than the diameter of the wafer 1 to allowtolerance for positioning the wafer in the pocket. Similar griddedsusceptors are commercially available from ASM America, Inc. of Phoenix,Ariz. for use in its Epsilon™ series of CVD reaction chambers.

[0011] In FIG. 1A, the wafer 1 is centered within the pocket such thatthere is uniform spacing between wafer edge 2 and shoulder edge 6throughout the wafer periphery. FIG. 1A represents the ideal position ofthe wafer 1 with respect to the susceptor 100. However, as shown in FIG.1B, upon initial placement the wafer 1 often tends to slide (upondrop-off) and/or jump (upon curl), and its outer edge 2 can contact orcome in close proximity to the inner edge 6 of the shoulder 4. Theshoulder 4 is thicker and thus generally cooler than the wafer 1 and theunderlying grid G. As a result, the portion of the edge 2 of the wafer 1in contact with the shoulder 4 tends to cool by conduction therebetween.This portion of the wafer edge 2 also tends to lose heat throughradiation if it is very near to the shoulder edge 6, even if the waferedge and the shoulder are not actually in contact.

[0012] Cooling at the wafer edge causes the temperature of the wafer tobe non-uniform. Since thin film deposition rates (and many otherfabrication processes) are often strongly temperature dependent,especially for CVD in the kinetic regime, film thickness and resistivitywill be non-uniform across a wafer processed under conditions oftemperature non-uniformity. Consequently, there is a need for animproved substrate support that facilitates substrate pick-up anddrop-off while promoting temperature uniformity.

SUMMARY OF THE INVENTION

[0013] In satisfaction of this need, the preferred embodiments of thepresent invention provide a substrate holder with gas flow to slow thedescent of a substrate thereabove. As the substrate descends slowly, thesubstrate temperature is permitted to increase by convection to anextent high enough to prevent extreme curl when the substrate eventuallymakes contact with the substrate holder. The gas flow also serves tocool the substrate holder, further reducing the temperature differencebetween the substrate and the substrate holder. When the substrate makescontact with the substrate holder, the temperature differencetherebetween is substantially reduced and/or eliminated, therebyreducing and/or eliminating thermal shock to the substrate. Theresulting reduction in substrate curl not only reduces damage tosubstrates and equipment, but also helps keep the substrate centered.Pick-up of the substrate is facilitated by providing a gas flow to helplift the substrate vertically off of the substrate holder. This alsoprevents stick. Stick can be further prevented by providing one or moregrooves and protrusions in the support surface of the substrate holder.Also, the substrate holder can be provided with centering means. Forexample, the holder can include outer gas passages for providingvertical gas jets just radially outward of the substrate periphery, forcounteracting sideward slide of the substrate during drop-off,processing, and/or liftoff. Gas passages and a gas supply system areconfigured to minimize stick, slide, and curl while still maintainingdesirable thermal properties. Methods for supporting a substrate on thesupport are also provided.

[0014] According to one aspect, the present invention provides asusceptor for supporting a wafer within a reaction chamber, comprisingan upper support surface configured to support a wafer, a plurality ofgas passages within the susceptor, and a gas supply system. The gaspassages have inlet ends configured to receive a gas flow from a sourceof gas, and outlet ends opening at the upper support surface. The gassupply system is configured to supply a generally upward flow of gasthrough the gas passages. The passages are configured so that such a gasflow would apply an upwardly directed force to a wafer above the uppersupport surface. The gas supply system is configured to supply a flowrate of gas sufficient to slow the rate of descent of a falling waferabove the upper support surface to a rate of descent no greater than onehalf of the rate at which the wafer would descend under gravity alone.

[0015] According to another aspect, the present invention provides asubstrate holder comprising a susceptor and a gas supply system. Thesusceptor includes a plurality of gas passages and a support surface.The gas supply system is configured to supply an upwardly directed flowof gas through the gas passages. The gas supply system is configured tosupply a flow rate of gas sufficient to slow the rate of descent of a100 mm substrate that is above and falling toward the support surface toa rate of descent no greater than one half of the rate at which the 100mm substrate would descend under gravity alone.

[0016] In some embodiments, the support surface of the susceptorincludes a plurality of grooves and protrusions, preferably forming acrisscross grid pattern. The substrate rests upon the tops of theprotrusions. During processing, the grooves permit sweep gas to flowunder the substrate and upward around a peripheral edge of the substrateto prevent the deposition of reactant gases on the underside of thesubstrate.

[0017] In some embodiments, wherein the susceptor is designed to hold asubstrate having a predetermined size, some of the gas passages aresubstantially vertically oriented and positioned so as to be justradially outward of a peripheral edge of a substrate of thepredetermined size and that is centered on the susceptor. When gas isdelivered upward through these outer gas passages, substantiallyvertical gas streams emerge above the susceptor. These gas streamsadvantageously counteract wafer slide during drop-off, processing, andliftoff.

[0018] In a preferred embodiment, the substrate holder further comprisesa spider assembly configured to support and preferably rotate thesusceptor. The spider assembly is hollow to permit the gas to flowupward through the spider assembly into the gas passages of thesusceptor.

[0019] According to yet another aspect, the present invention provides amethod of supporting a substrate, as follows. A substrate is releasedabove a support surface of a susceptor, such that the substrate ispermitted to descend toward the support surface by gravitational force.A cushioning flow of gas is provided, which imparts an upwardly directedforce onto the substrate. The flow rate of the cushioning gas flow issufficient to slow the rate of descent of the substrate to a rate nogreater than one half of the rate at which the substrate would descendunder gravity alone. The substrate is then permitted to come intocontact with the support surface. In one embodiment, the cushioning flowof gas is provided upwardly through a plurality of gas passages providedin the susceptor.

[0020] The method may further comprise providing a “trickle” flow of gasthrough the plurality of gas passages after the substrate comes intocontact with the support surface of the susceptor, to prevent thedeposition of reactant gases onto the underside of the substrate duringprocessing. The support surface preferably has a plurality of groovesextending radially outward beyond a peripheral edge of the substrate.The flow rate of the trickle gas flow is sufficient to cause the gas toflow under the substrate through the grooves and upward around theperipheral edge of the substrate without disrupting contact between thesubstrate and the support surface.

[0021] The method may further comprise lifting the substrate after it isin contact with the support surface of the susceptor. Accordingly, a“liftoff” flow of gas is provided through the plurality of gas passages.The liftoff gas flow can be sufficient to lift the substrate off of thesupport surface, or it can simply aid in lifting the substrate. Thesubstrate is then removed from above the support surface.

[0022] The method may further comprise providing gas flow through outergas passages of the susceptor, to counteract sideward sliding of thesubstrate. The outer gas passages are positioned just radially outwardof a peripheral edge of the substrate when the substrate is centeredupon the support surface of the susceptor. The outer gas passages aresubstantially vertical so that upward gas flow through the outer gaspassages flows substantially vertically upward above the susceptor. Gascan be supplied to the outer gas passages during wafer drop-off,processing, and/or liftoff.

[0023] In another aspect, the invention provides a method of loading awafer onto a susceptor. According to the method, a wafer is releasedabove a susceptor, the wafer having a peripheral edge. A plurality ofsubstantially vertical upwardly directed gas jets are provided radiallyexterior of the peripheral edge of the wafer. The jets substantiallyinhibit sideward motion of the wafer as the wafer descends toward thesusceptor.

[0024] For purposes of summarizing the invention and the advantagesachieved over the prior art, certain objects and advantages of theinvention have been described herein above. Of course, it is to beunderstood that not necessarily all such objects or advantages may beachieved in accordance with any particular embodiment of the invention.Thus, for example, those skilled in the art will recognize that theinvention may be embodied or carried out in a manner that achieves oroptimizes one advantage or group of advantages as taught herein withoutnecessarily achieving other objects or advantages as may be taught orsuggested herein.

[0025] All of these embodiments are intended to be within the scope ofthe invention herein disclosed. These and other embodiments of thepresent invention will become readily apparent to those skilled in theart from the following detailed description of the preferred embodimentshaving reference to the attached figures, the invention not beinglimited to any particular preferred embodiment(s) disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1A is a schematic cross-sectional view of a wafer centered ona conventional susceptor;

[0027]FIG. 1B is a schematic cross-sectional of a wafer positioned offcenter on the conventional susceptor of FIG. 1A;

[0028]FIG. 2 is a schematic, cross-sectional view of an exemplaryreaction chamber with a wafer supported on a susceptor therein;

[0029]FIG. 3 is a side cross-sectional view of a controlled gas cushionsusceptor system constructed according to a preferred embodiment of thepresent invention;

[0030]FIG. 4A is a top plan view of a susceptor similar to that of FIG.3, illustrating an arrangement of the gas passages, in which the wafersupport grid is not shown for clarity and ease of illustration;

[0031]FIG. 4B is a top plan view of the susceptor of FIG. 3,illustrating inner gas passages as well as outer gas passages at theedges of the wafer, in which the wafer support grid is not shown forclarity and ease of illustration;

[0032]FIG. 5 is a top plan view of a lower section of the susceptor ofFIG. 4A, illustrating an arrangement of grooves for the passage of gasinto the gas passages of FIG. 4A;

[0033]FIG. 5A is an exploded view of a portion of the lower susceptorsection of FIG. 5, illustrating the relationship between the spiderarm-receiving recesses on the bottom surface of the section and thegrooves on the top surface of the section;

[0034]FIG. 6 is a top plan view of an upper section of a susceptorconstructed according to a preferred embodiment of the presentinvention, the upper section having a gridded pocket for holding awafer;

[0035]FIG. 7A is a partial cross-sectional view of the susceptor uppersection of FIG. 6, taken along line 7A-7A thereof;

[0036]FIG. 7B is an enlarged cross-sectional view of a portion of thesupport grid of FIG. 7A;

[0037]FIG. 8A is a schematic perspective and partially cut away view ofa wafer supported on the susceptor of FIG. 7A; and

[0038]FIG. 8B is an enlarged cross-sectional view of the circled regionin FIG. 8A showing the support grid beneath a perimeter of the wafer.The cross section follows the curve of the wafer edge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] As noted above, there are significant problems associated withthe transfer of substrates onto and from conventional substrate holders.In dropping substrates onto the holders, substrate slide makes itdifficult to accurately place the substrate in the center of the holderwith good reproducibility. When the holder is heated, particularly whenthe holder is a heated susceptor in a cold-wall reactor, a substratedropped onto the holder also tends to curl due to transitory temperaturedifferentials within the substrate. Curl can cause “jump” and move thesubstrate from its desired position. Due to unpredictable placement ofthe substrate upon the susceptor, it is difficult to maintain a uniformsubstrate temperature, especially for processes within the kineticregime.

[0040] Furthermore, curl can cause scratching of the wafer-handling endeffector and dropping of the substrate, leading to particulatecontamination. Also, in removing the wafer from the holder, the wafertends to stick to the holder (known as “stick” or “stiction”).Sometimes, the wafer lifts the holder and drops it back onto thesupporting structure, causing further particle generation. Theseparticle problems can cause contamination of whole wafers or evenbatches of wafers, significantly reducing yield.

[0041]FIG. 2 illustrates a cold wall reactor chamber 20 for processingsemiconductor wafers, within which a gas cushion susceptor system 22 ofthe present invention is incorporated. Prior to discussing the detailsof the gas cushion susceptor system 22, the elements of the reactionchamber 20 will be described. Although the gas cushion susceptor system22 is preferably incorporated into the illustrated reaction chamber 20,the system 22 is suitable for many different types of wafer processingsystems, and the discussion herein should not be limited to oneparticular type of reaction chamber. In particular, one of ordinaryskill in the art can find application for the gas cushion susceptor andsubstrate support method described herein for other semiconductorprocessing equipment. Moreover, while illustrated in the context ofstandard silicon wafers, the substrate supports described herein can beused to support other kinds of substrates, such as glass, which aresubjected to treatments such as CVD, physical vapor deposition (PVD),etching, annealing, dopant diffusion, photolithography, etc. Thesubstrate supports of this invention are of particular utility forsupporting substrates during treatment processes at elevatedtemperatures, and even more particularly for systems in which coldwafers are loaded onto hot supports.

[0042] The chamber 20 comprises a quartz tube defined by an upper wall24, a lower wall 26, an upstream flange 28, and a downstream flange 30.Although not shown in the figure, lateral edges of the reaction chamber20 include relatively thick side rails between which a chamber dividerplate 32 is attached. FIG. 2 is a longitudinal cross-section along acentral vertical plane of the chamber 20 illustrating the verticaldimension of the lenticular shape; the side rails are thus not seen.Preferably, the chamber 20 is manufactured from quartz. The chamberdivider plate 32 reinforces the chamber 20 during vacuum processing andextends between the side rails (not shown), preferably along the centerline of the chamber 20. The divider plate 32 includes an aperture 33defining a void or opening 35 extending across the lateral dimension ofthe chamber 20 between the side rails. The aperture 33 divides thedivider plate 32 into an upstream section extending from the upstreamflange 28 to an upstream edge of the aperture, and a downstream sectionextending from a downstream edge of the aperture to the downstreamflange 30. The upstream section of the divider plate 32 is preferablyshorter in the longitudinal direction than the downstream section.

[0043] An elongated tube 34 depends from a centrally located region ofthe lower wall 26. A drive shaft 36 extends through the tube 34 and intoa lower region 38 of the chamber 20. The lower region 38 is definedbetween the central chamber divider plate 32 and the lower wall 26. Theupper end of the drive shaft 36 is tapered to fit within a recess of amulti-armed support or spider assembly 40 for rotating a segmentedsusceptor 42. The susceptor 42 supports a wafer 44. A motor (not shown)drives the shaft 36 to, in turn, rotate the gas cushion susceptor system22 and wafer 44 thereon within the aperture 33. A gas injector 46introduces process gas, as indicated by arrow 48, into an upper region50 of the chamber 20. The upper region 50 is defined between the upperwall 24 and the chamber divider plate 32. The process gas passes overthe top surface of the wafer 44 to deposit chemicals thereon. The systemtypically includes a plurality of radiant heat lamps arrayed around theoutside of the reaction chamber 20 for heating the wafer 44 andcatalyzing the chemical deposition thereon. An upper bank of elongatedheat lamps 51 is illustrated outside of the upper wall 24, and typicallya lower bank of lamps (not shown) arranged cross-wise to the upper bankis also utilized. Further, an array of spot lamps directed upward fromunderneath the susceptor 42 is often used.

[0044] A source of gas 37 is schematically shown connected through amass flow controller 39 to the drive shaft 36. This gas source ispreferably provided with the ability to control the temperature of thegas via heaters or the like which are not depicted in the figure. Gasflows into the space within the hollow shaft 36 and is eventuallydirected upward through the susceptor 42, as will be more fullydescribed below. The fluid coupling allowing gas to the interior of thehollow, rotating shaft 36 is not shown, but may be accomplished by anumber of different means, one of which is shown and described in U.S.Pat. No. 4,821,674, the entire disclosure of which is herebyincorporated herein by reference.

[0045] A wafer is introduced to the reaction chamber 20 through a waferentry port 47. The wafer is typically transported by a robot pick-up arm(not shown) which enters through the port 47 and extends over the wafersupport system 22 to deposit the wafer thereon. The CVD system thenseals the reaction chamber 20 and introduces process gas for depositinga layer of silicon or other material on the wafer. After processing, agate valve opens and the robot pick-up arm enters through the port 47and retracts the wafer from the susceptor 42. Periodically, the reactionchamber 20 is preferably conditioned for subsequent processing. Atypical sequence is the introduction of an etch gas into the reactionchamber with the gate valve closed to clean any particular depositionfrom the support structures and interior walls. After the etching, asilicon precursor is sometimes introduced into the chamber to provide athin coat of silicon on the susceptor 42. Such a coating step issometimes termed capping and serves to stabilize emissivity of thesusceptor over repeated deposition cycles. After the etching and cappingsteps, the chamber is purged with hydrogen and heated for introductionof the next wafer.

[0046] The tube 34 is sized slightly larger than the drive shaft 36 toprovide space therebetween through which purge gas 52 flows. The purgegas enters the lower region 38 of the reaction chamber 20 to helpprevent reactant gas from depositing in the lower region. In thisrespect, the purge gas 52 creates a positive pressure below the wafersupport system 22, which helps prevent reactant gas from travelingaround the sides of the segment susceptor 42 in the lower region 38. Thepurge gas is then exhausted, as indicated with arrows 54 and 55, betweenthe susceptor 42 and aperture 33 into the upper region 50 and thenthrough an elongated slot 60 in the downstream flange 30. This ensuresthat reactant gases do not migrate into the lower region 38. The purgegas continues through an exhaust system 58. Excess reactant gas andreaction by-product likewise passes through the elongated slot 60 in thedownstream flange 30 to be vented through the exhaust system 58.

[0047] Preferably, a temperature compensation ring 62 surrounds thewafer support system 22. The ring 62 fits in the opening 35 created bythe aperture 33 in the divider plate 32, and the wafer support system 22and ring 62 together substantially fill the opening and providestructure between the lower and upper chamber regions 38, 50. Thesusceptor 42 rotates within the ring 62 and is preferably spacedtherefrom across a small annular gap of between 0.5 and 1.5 mm. In thecase of a ring 62 having a circular outer periphery, the shape of theaperture 33 in the divider plate 32 surrounding the ring 62 can be madecircular so that the edges of the opening 35 are in close proximity tothe ring. Alternatively, the ring 62 may have a rounded rectangularouter periphery. As will be described in greater detail below, thesusceptor 42 is preferably manufactured to have a constant outerdiameter to fit within the ring 62. Although the susceptor 42 has aconstant outer diameter, it will be seen that various configurations areprovided for processing a number of different size wafers.

[0048] In a particularly advantageous embodiment, the temperaturecompensation ring 62 comprises a two-part circular ring having a cavitytherein for receiving thermocouples 64. In the illustrated embodiment,the thermocouples 64 enter the chamber 20 through apertures formed inthe downstream flange 30 and extend underneath the divider plate 32 intothe temperature compensation ring 62. The apertures in the quartz flange30 substantially prevent gas leakage around the thermocouples 64,although typically no additional seal is used. There are preferablythree such thermocouples, one 66 terminating at a leading edge, one 68terminating at a trailing edge, and one (not shown) terminating ateither of the lateral sides of the ring 62. The thermocouples within thering 62 surrounding the segmented susceptor 42 provide good temperatureinformation feedback for accurate control of the radiant heating lamps.A plurality of bent fingers 70 attached to the divider plate 32 supportthe ring 62 around the periphery of the susceptor 42. In addition to thetemperature compensation ring 62 and thermocouples therein, a centralthermocouple 72 extends upward through the drive shaft 36, which isconstructed hollow, and through the spider assembly 40 to terminateunderneath the center of the susceptor 42. The central thermocouple 72thus provides an accurate gauge of the temperature near the center ofthe wafer 44.

[0049] In addition to housing the thermocouples 64, the temperaturecompensation ring 62 absorbs radiant heat during high temperatureprocessing. This compensates for a tendency toward greater heat loss atthe wafer edge, a phenomenon that is known to occur due to a greaterconcentration of surface area for a given volume near such edges. Byminimizing edge losses and the attending radial temperaturenon-uniformities across the wafer, the temperature compensation ring 62can help to prevent crystallographic slip and other problems associatedwith temperature non-uniformities across the wafer. The temperaturecompensation ring 62 can be suspended by any suitable means. Forexample, the illustrated temperature compensation ring 62 rests uponelbows 70, which depend from the quartz chamber dividers 32.

[0050] Now, referring to FIG. 3, a preferred embodiment 22 of a gascushion susceptor system according to the present invention is shown.Again, the system 22 generally comprises the susceptor 42 supported byarms 74 of the spider assembly 40. The arms 74 extend radially outwardfrom a hub 76 and bend vertically upward at predetermined radialdistances to contact the underside of the susceptor 42. For ease ofmanufacture and assembly, the illustrated susceptor 42 comprises anupper section 78 and a lower section 80, both sections being generallyplanar disk-shaped elements. Both sections 78, 80 of the susceptor 42are preferably machined out of graphite and fit closely together withoutadditional fastening means to ensure minimal gas leakage therebetween. Agap of less than 0.001 inches between either or both of the adjacentvertical circular interface and the flat horizontal interface betweenthe upper and lower sections 78, 80 is acceptable for this purpose. Athin coating of silicon carbide is preferably formed on susceptor piecesmachined from graphite. In one embodiment, the thickness of thesusceptor 42 is about 0.30 inches. The thickness will depend on theoverall size of the susceptor.

[0051] The upper section 78 generally comprises an outer ring 82surrounding a thinner circular middle portion 83. The outer ring 82comprises an upper rim or ledge 84 and a lower rim or skirt 86, whichterminate at upper and lower shoulders or steps 88 and 90, respectively.The upper step 88 forms a transition between the ledge 84 and thecircular middle portion 83. Together, the step 88 and the middle portion83 define a circular wafer-receiving recess 92. The lower step 90 formsa transition between the skirt 86 and the middle portion 83. Togetherthe step 90 and the middle portion 83 define an annular recess 94 in theunderside of the upper section 78. The annular recess 94 is sized toreceive the lower section 80. The lower section includes spiderarm-receiving recesses 214 for receiving the upper ends of the spiderarms 74, permitting the spider assembly to provide stable support to thesusceptor 42.

[0052] An exemplary design for a two-piece susceptor allowing for gasflow through the susceptor and underneath the wafer to preventdeposition on the lower surface of the wafer is illustrated anddescribed in U.S. Pat. No. 6,053,982, the entire disclosure of which isincorporated herein by reference.

[0053] In a preferred embodiment of the invention, a cushion gas flowsupward at a constant or variable flow rate from a gas source below thewafer support system 22 through the spider assembly 40 and through thesusceptor 42 to the underside of the wafer. While a constant flow rateof cushion gas may be preferred, the flow rate may be varied as thewafer descends onto the susceptor. The cushion gas may be providedduring wafer drop-off, wafer processing, or wafer lift-off. The cushiongas may be the same as the purge gas 52 described above. Alternatively,the cushion gas may be different from the purge gas. In one embodiment,the cushion gas comprises a mixture of N₂ and H₂ gases.

[0054] In the embodiment illustrated in FIG. 3, the spider assembly 40receives a flow 109 of the cushion gas within the hub 76. The cushiongas flows through the arms 74 of the spider assembly, as depicted byarrows 111, into passages 103 opening at the bottom surface of the lowersection 80. The passages 103 are fluidly connected to the recesses 214so that gas flowing upward through the spider assembly freely flows intothe passages 103. The passages 103 extend upward into an upper set ofinterconnected grooves or recesses 105 in the top surface of the lowersection 80. The grooves 105 provide a conduit between the passages 103to a plurality of passages 96 and 97 within the upper section 78. Theskilled artisan will appreciate that the passages 103 and the grooves105 can have any of a variety of different configurations, keeping inmind the goal of delivering the cushion gas to passages 96 positionedthroughout the upper section 78 of the susceptor 42.

[0055] The upper section 78 further includes a plurality of gas passagepassages 96. In the figure, a limited number of such passages aredepicted. However, in practice a much greater number of passages 96 willgenerally be used, which passages will be much smaller than those shown.For the sake of clarity, the various features have been shown inexaggerated form in the drawing. For a susceptor 42 adapted to support a200 mm diameter wafer, there are preferably at least twenty passages 96,more preferably about sixty four passages 96, and even more preferablyabout 80 passages 96. With a larger wafer there are preferably even morepassages. Preferably, the passages 96 are distributed substantiallyuniformly throughout the top surface of the upper section 78 of thesusceptor 42 which contacts the wafer. Such a distribution minimizes therisk of uneven cooling of the wafer from gas flowing upward through thepassages 96, which could induce slip. The passage diameter is preferablyno greater than about 2 mm, and preferably is about 0.25 mm. In someembodiments, the gas flowing upward through the passages 96 may beheated to reduce the cooling effect on the wafer. Preferably, thepassages 96 span substantially the entire upper surface of thewafer-receiving recess 92 of the susceptor, to reduce the likelihood oflocalized over-cooling of the wafer.

[0056] In the illustrated embodiment, the passages 96 are orienteddiagonally so that the cushion gas flows upward and radially outwardfrom the top surface of the susceptor toward the underside of the wafer,as depicted by the arrows 113. Such an orientation of the passages 96facilitates the prevention of “backside deposition” during waferprocessing. A small upward “trickle” gas flow through the passages 96flows radially outward between the susceptor and the wafer and upwardaround the peripheral edges of the wafer to prevent reactant gases fromdepositing on the underside or “backside” of the wafer during waferprocessing. Preferably, the passages 96 are oriented at an angle of atleast 10° from vertical, and more preferably at least 45° from vertical.The outer passages 97 are preferably substantially vertical (preferablywithin 25° from vertical) so that gas expelled therefrom flowssubstantially vertically upward, as depicted by arrows 107. As explainedin greater detail below, the outer passages 97 serve to inhibit andpreferably prevent wafer slide.

[0057] The positioning and number of the passages 96 within thewafer-receiving recess 92 should be such as to effect a gas-cushioneddescent of the substrate during wafer load, and to facilitate liftoff ofthe wafer during its removal via upward gas flow through the passages96. FIG. 4A is a top view of a susceptor having an exemplary pattern ofpassages 96 without the passages 97. The preferred grid structure is notshown for ease of illustration. In the illustrated embodiment, thepassages 96 are positioned along twelve lines extending radially outwardfrom the center of the susceptor and separated angularly by 30°increments. However, those of ordinary skill in the art will appreciatethat many different arrangements of passages 96 can be provided.

[0058] Preferably, the gas passages 96 of the upper section 78 of thesusceptor 42 are positionally balanced with respect to the supportsurface of said susceptor. In other words, the locations of the variousgas passages 96 are preferably distributed in a radially symmetricmanner, such that an upward flow of gas through the passages causessubstantially balanced upward force upon a substrate above the supportsurface of the susceptor. In contrast, if the passages 96 wereconcentrated toward one half of the support surface, the flow of gascould cause the substrate to tilt or even flip over.

[0059]FIG. 5 shows a top view of a lower section 80 in accordance withthe susceptor of FIG. 4A. The lower section 80 includes a set ofinterconnected grooves 105 in its top surface. In the illustratedembodiment, the grooves 105 comprise twelve groove portions spaced apartby angles of 300, to match the specific arrangement of gas passages 96shown in FIG. 4A. An inner circular groove 217 and an outer circulargroove 211 connect all of the groove portions 105, so that gas flowssubstantially throughout all of the groove portions. Those of ordinaryskill in the art will understand that many different arrangements ofpassages 96 and grooves 105 are possible. In the illustrated embodiment,three of the groove portions 105 align with recesses 214 in the bottomsurface of the lower section 80, the recesses 214 configured to receivethe upper ends of the support arms 74 of the spider assembly. FIG. 5Aillustrates in clearer detail the configuration of the recesses 214. Asshown, the recesses 214 are aligned with inner vertical passages 103. Inthe illustrated embodiment, gas from the spider assembly flows throughthe vertical passages 103 into the grooves 105, 211, and 217. The uppersection 80 has a central hole 215 for receiving the central spindle 102(FIG. 3).

[0060]FIG. 4B illustrates a susceptor having the additional outerpassages 97 in addition to the passages 96. The outer passages 97 aresubstantially vertical so as to expel gas jets or streams verticallyupward. The outer passages 97 are positioned just outside of the outerperiphery of the wafer when the wafer is held within the pocket of thesusceptor. In the illustrated embodiment, the outer passages arepositioned within the annular groove 98 at the periphery of the wafer.These outer passages 97 serve to inhibit and preferably to prevent waferslide during load and removal of the wafer, as well as to preventdeposition of process gases on the backside of the wafer duringdeposition. During wafer drop-off or removal, the jet streams flowingthrough the passages 97 counteract sideward movement of the wafer.

[0061] The skilled artisan will understand that it may be desirable toprovide a degree of “baffling” of the gas flow before it exits the gaspassages 96 and, if provided, 97. In other words, it may be desirable toextend the path of gas flow within the susceptor, from the point atwhich the gas exits the spider arms 74 into the susceptor and the pointat which the gas exits the passages into the region above the susceptor.This extension of the gas flow path improves the uniformity of gas flowthrough the passages 96 and 97, thus providing a more balanced forceonto the descending substrate above the susceptor and preventinglocalized overcooling of the substrate. An exemplary design of atwo-piece susceptor including a “baffling” of the gas flow within thesusceptor is illustrated and described in U.S. Pat. No. 6,053,982,incorporated herein by reference.

[0062] In a preferred embodiment, as shown in FIGS. 3, 6, 7A, and 7B anddescribed in further detail below, the recess 92 of the susceptor 42 hasa grooved surface with a plurality of projections to reduce thepotential for wafer sticking or sliding. The upper section 78 furtherincludes a downwardly depending central spindle 102 defining a radiallyinner border 123 of the annular lower recess 94. A central thermocouplecavity 104 is defined in the spindle 102 for receiving a sensing end ofthe central thermocouple 72 (FIG. 2) previously described. The spiderassembly 40 having curved arms, which is depicted in FIGS. 2 and 3, ispreferentially employed. Alternatively, a spider having tubes bent atsharp right angles may be employed. Spider assemblies that may beemployed with the susceptors of the present invention are disclosed inU.S. Pat. No. 6,203,622, the entire disclosure of which is herebyincorporated herein by reference.

[0063] Details of the surface of the wafer holder of the preferredembodiment will now be shown with reference to FIGS. 6-8. As notedabove, the illustrated gas cushion susceptor system has a susceptor 42capable of absorbing radiant energy from the heating elements 51 (FIG.2). The susceptor is preferably made of graphite coated with siliconcarbide, although the skilled artisan will appreciate that othermaterials can also be used. The illustrated susceptor is of a typeconsiderably more massive than the wafer to be supported, preferablymore than five times and more preferably between about 7 and 9 times thethermal mass of the wafer, such that it can serve as a “thermalflywheel” to maintain temperature stability.

[0064]FIG. 6 shows a preferred embodiment of the upper section 78 of thesusceptor 42 as viewed from the top, that is, looking into a recessedpocket 92 in which the wafer will be supported. The recessed pocket 92has a set of perpendicular, crossing grid grooves 222 cut into itssurface and surrounded by an annular groove 98. These features will bedescribed in more detail with respect to FIG. 7A below. While shownacross only a portion of the susceptor pocket 92, it will be appreciatedthat the grid extends across the full susceptor pocket 92 up to theannular groove 98. The outer ring 82 and the raised shoulder or step 88surrounds the annular groove 98. In one embodiment, the susceptor 42 isdesigned to support a 200 mm wafer, and the diameter of the uppersection 78 to the outer edge of the annular groove 98 is about 8.000inches, or slightly larger than the wafer to be supported. In thisembodiment, the overall diameter of the upper section 78 (and hence thesusceptor) is about 8.850 inches.

[0065]FIG. 7A is a cross-sectional view of an area near the periphery ofthe upper section 78 of the susceptor, along the line 7A-7A in FIG. 6.On the top surface, the pocket 92 is shown with a plurality of gridprotrusions 220 separated by a plurality of parallel grid grooves 222,perpendicular to the plane of the figure. The skilled artisan willappreciate that there is a second set of similar grid grooves (notvisible in this view), perpendicular to the illustrated grid grooves 222and parallel to the plane of the figure. Thus the protrusions 220 can beunderstood as small, square islands, bordered on two parallel sides byone set of grid grooves 222 and on the other two parallel sides by thesecond set of grid grooves not seen in this view. The annular groove 98,the susceptor shoulder 88, the outer ring 82, and the relative positionsthereof are also shown. The grid protrusions 220 have top surfaces 228.The outer ring 82 has a top surface 89.

[0066]FIG. 7B is a detail of the pocket 92 surface shown in the circlelabeled 7B in FIG. 7A. Each grid groove 222 has a flat grid floor orbottom surface 224 and sidewalls 226 that slant upward and outwardtherefrom. The protrusions 220 between the grid grooves 222 have flattop surfaces 228 that define the support surface of the pocket 92. Inthe illustrated embodiment, for a susceptor sized to hold a 200 mmwafer, the surfaces 228 are square with a width and length of about0.008 inches by 0.008 inches (0.20 mm by 0.20 mm), while the flat bottomsurfaces 224 of the grid grooves 222 are about 0.0221 inches (0.56 mm)in width. These numbers will preferably be different for different sizewafers.

[0067] The difference in height between the protrusion top surfaces 228and the grid groove bottom surfaces 224 is preferably between about 0.35mm and 0.55 mm, and more preferably between about 0.40 mm and 0.45 mm(nominally 0.43 mm or 0.017 inches in the illustrated embodiment). Thepitch of the grid, or distance between identical adjacent features, ispreferably between about 1.0 mm and 1.5 mm, more preferably betweenabout 1.2 mm and 1.3 mm in both directions (nominally 1.27 mm or 0.050inches in the illustrated embodiment).

[0068] Similar gridded susceptors have been available from ASM America,Inc. of Phoenix, Ariz. for use in the Epsilon™ series of CVD reactors.Those susceptors, however, had different grid configurations. Forinstance, the pitch of the grid in prior susceptors was about half thatof the preferred embodiment. The upper support surface of the susceptorof the preferred embodiment is designed to be nearly planar, with theexception of a minimal manufacturing tolerance for concavity (e.g., from0 to 0.005 inches or 0.127 mm, for a susceptor designed to hold a 200 mmwafer), as compared to the peripheral portions of the grid, to avoid aconvex shape. In other words, in as far as it is not possible to providea perfectly flat wafer support surface, it is preferred that the surfacebe slightly concave with respect to the wafer rather than convex. Aconcave configuration promotes stability and balance, since the waferwill be supported at its periphery. In contrast, a convex susceptor willsupport the wafer only at the center, causing the wafer to be unstableand exacerbating thermal gradients on drop-off.

[0069]FIG. 8A is a perspective view of the substrate or wafer 44 inposition on a susceptor upper portion 78 according to a preferredembodiment of the present invention. The cut-away portion shows the edgeof the wafer 44 overlying protrusions 220 at or near the periphery ofthe susceptor pocket. The scale of the grid is exaggerated for ease ofillustration.

[0070]FIG. 8B shows the wafer edge in contact with the grid protrusions220 at the outer edge of the susceptor pocket and viewed edge on. Thegrid is sectioned along a line of constant radius near the wafer edge orperimeter. This section intercepts a plurality of grid grooves 222, asshown. The cross section thus depicts the openings of grid grooves 222at or near the wafer edge. Gas (e.g., air or inert gas in the chamber,as well as gas supplied through gas passages 96) flows radially outwardthrough the grooves 222 under the wafer during wafer drop-off andradially inward through the grooves 222 during wafer pick-up.

[0071] Those of ordinary skill in the art will appreciate that many ofthe advantages of the present invention can be obtained without the gridstructure on the support surface of the susceptor 42. In other words, ifdesired, the susceptor can be modified so that it does not include theprotrusions 220 and the grooves 222. Instead, the support surface can besubstantially flat.

[0072] The present invention includes a method of providinggas-cushioned support of a semiconductor substrate, utilizing theapparatus described above. A substrate is first brought to a positionabove the susceptor 42 by an end effector. When the substrate is broughtabove the susceptor 42, gas is supplied from gas source 37, throughhollow shaft 36 and spider assembly 40, and is supplied to gas passageholes 96. At this point, the flow rate of the supplied gas (termed the“cushion gas flow”) is preferably sufficient to slow the descent of thesubstrate after release by the end effector to a rate (termed the“cushioned descent rate”) significantly less than that at which thesubstrate would descend if it were simply dropped onto the susceptor 42under substantially only the influence of gravity with no cushion gasflow (termed the “unimpeded descent rate”). The gas flow rate depends onthe dimensions of the susceptor, loss of flow caused by contact betweenthe susceptor support and susceptor, the size and weight of thesubstrate, the type of gas, and the amount of flow needed to cool thesusceptor. Preferably, the cushioned descent rate is no greater thanhalf the unimpeded descent rate, more preferably no greater thanone-third the unimpeded descent rate, and most preferably no greaterthan one-quarter the unimpeded descent rate. The cushion gas flow ratemay be changed during substrate descent. For example, the flow rate maybe increased to decrease the cushioned descent rate as the substrateapproaches the upper surface of the susceptor. The preheat time is thetime lapse between the releasing of the substrate above the supportsurface of the susceptor and the moment the substrate contacts thesupport surface. Typically, the susceptor has a temperature within therange of 200-1000° C., and the substrate has a temperature within therange of 0-100° C. at the time of the releasing of the substrate abovethe susceptor.

[0073] In a system configured to modularly accept multiple wafer sizes,the gas supply system is preferably configured so that the flow rate ofthe gas is controllable and variable from zero to a flow rate sufficientto apply an upwardly directed force to a wafer to levitate the waferabove the support surface of the susceptor. For a 200 mm wafer, the gassupply system is preferably configured to provide a flow rate of H₂ gasof at least 15-20 slm.

[0074] The cushion gas flow advantageously reduces namely wafer curlresulting from disparities in temperature in the wafer. When initiallyintroduced into the chamber and held over the susceptor, a wafer isheated disproportionately from below. Accordingly, the highertemperature on the bottom surface of the wafer results in greaterthermal expansion on the bottom surface and, therefore, a slight amountof upward curl. The wafer has a tendency to assume a bowl-like orconcave shape, with a concavity on the order of about 0.010 inches.Concavity in this context refers to the depth from the highest point ofthe wafer (generally the edge) to the lowest point of the wafer(generally the center). If the concave wafer 44 is simply dropped onto aflat gridded susceptor without a cushion gas flow, the center of thewafer is the first portion thereof to contact the susceptor, whichintroduces radial temperature gradients. This quickly exacerbates thecurl effect, such that the concavity increases to about 0.350 inch uponcontact, often scratching the end effector before it can be withdrawnand sometimes resulting in breaking of the wafer.

[0075] The cushion gas flow can be advantageously used to counteractthis effect. As noted above, the gas source 37 is provided with theability to control the temperature of the gas supplied through the gaspassage holes. The cushion gas flow slows the rate of descent of thesubstrate, allowing the susceptor and/or lamps to preheat the substrateprior to contacting the hot susceptor. In some susceptors according tothe invention (such as that of FIG. 4B, which includes peripheralvertical outer passages 97), the cushion gas flow also preventsmisalignment due to sliding or skating. The control of the rate ofdescent permits greatly improved control of the temperature of thesubstrate. By supplying heated gas through the gas supply holes, thetemperature of the descending substrate is brought to a temperatureapproaching that of the susceptor in a relatively gradual manner beforethe substrate makes contact with the susceptor. As noted above, thesudden contact between a substrate and a much hotter susceptor, and theresulting disparity in temperature across the substrate, can cause waferjump and is a primary cause of wafer curl. Wafer jump and curl can causefurther damage to the end effector on wafer pick-up, particularly when aBernoulli wand-type end effector is employed. However, when a cushiongas flow is employed as described herein, it is possible to bring thesubstrate to a higher temperature prior to contact with the susceptorand thus to avoid extreme wafer curl. Prior to contacting the susceptor,the temperatuer difference between the substrate and the susceptor ispreferably no greater than 100° C. The use of the cushion gas flow inthis manner reduces wafer curl, which can be as great as 0.350 inchesconventionally, to no more than 0.200 inches, more preferably no morethan 0.100 inches, and most preferably no more than 0.050 inches.

[0076] As shown in FIG. 4B, in one embodiment vertical outer passages 97are provided as part of the cushion gas flow, further counteractingphenomena that tend to dislocate the substrate, such as skating. Inparticular, upward vertical streams of gas emerging from the openings ofthe outer passages 97 during wafer drop-off and pick-up counteracts thetendency of the wafer to slide horizontally. The gas flow through theouter passages 97 also helps prevent reactant gases from depositing onthe underside of the substrate during wafer processing. This part of themethod is discussed below.

[0077] After the substrate has descended to a point at which it is incontact with the susceptor over substantially the entire bottom surfaceof the substrate, the cushion gas flow is preferably reduced to a muchlower flow rate. This gas flow rate is preferably insufficient todisturb contact between the projections 220 of the susceptor and thesubstrate. This “trickle” or “sweep” gas flow passes through the grooves222 provided in the surface of the susceptor and below the bottomsurface of the substrate, and exits at the edge of the substrate,passing upward into the processing chamber. This trickle gas flowadvantageously serves to inhibit the flow of process gases from abovethe substrate to the area below the substrate, thus inhibiting undesireddeposition of process gases onto the bottom surface of the substrate.For H₂ gas, the trickle gas flow rate from the gas supply system ispreferably within the range of 5-10 slm. Preferably, the ratio of thecushion gas flow rate to the trickle gas flow rate is within the rangeof 1.0 to 2.5.

[0078] After the processing of the semiconductor substrate is completed,the cushion gas flow can be increased to a level sufficient to aid liftof the substrate off of the susceptor. As noted above, substrates areprone to sticking on removal from conventional wafer supports because ofthe vacuum effect caused by an excessively slow flow of gas into thesmall space between the wafer and the surface of the pocket of the waferholder. However, the cushion gas flow of the present inventioneliminates this problem, as gas is supplied directly to the space belowthe substrate through the plurality of gas passages 96. The cushion gasflow aids in lifting the wafer off of the susceptor and allows the endeffector to more reliably lift the substrate. The reduction andpreferable prevention of stick reduces contamination, by preventingparticles from dropping off of the substrate onto the susceptor. It willbe understood that it is not necessary to provide a gas flow duringlift-off, as the grooves 222 in the susceptor support surface alone tendto reduce the likelihood of stick upon lift-off.

[0079] Thus, it is clear that the cushion gas flow can be providedduring (1) wafer drop-off, (2) wafer processing, and (3) wafer lift-off.Those of ordinary skill in the art will understand that the cushion gasflow can be provided during only one or two of these three processes, inany combination. For example, the cushion gas flow can be provided onlyduring wafer drop-off and lift-off. It is not necessary to provide acushion gas flow during wafer processing. Alternatively, the cushion gasflow can be provided only during wafer drop-off. In yet anotheralternative, the cushion gas flow can be provided only during waferliftoff, or during processing as well as liftoff. The skilled artisanwill appreciate that other combinations are possible.

[0080] Furthermore, where substantially vertical gas jets are suppliedfrom outer gas passages 97 as shown in FIG. 4B, the wafer may be morereliably centered during drop-off. While more preferred during drop-off,the substantially vertical jets from the passages 97 may also beprovided when the wafer is seated on the susceptor, as a furtherprotection against wafer slide. The vertical gas jets also can be usedwithout the remaining cushion gas flow, in an appropriately designedsusceptor. In other words, a susceptor can be formed with the outerpassages 97 but not the passages 96.

[0081] Depositions performed on a wafer supported by a gridded susceptorand employing the cushion gas support method in accordance with thepreferred embodiments will result in improved reproducibility and lowstandard deviation in deposited layer thickness. Moreover, improvedcontrol over drop-off should also help avoid failure in processing.

[0082] Those of ordinary skill in the art will understand that thesusceptors of the present invention can be modified to remove thegridded top surface. In other words, the pocket that receives the wafercan have a substantially flat surface without the protrusions 220 andgrooves 222 (FIG. 7B). In this embodiment, the gas passages 96 are stillprovided.

[0083] While a two-piece susceptor is easier to construct, the susceptorcould be implemented in a one-piece design. One method of manufacturingsuitable gas passages within a one-piece susceptor is disclosed in U.S.Pat. No. 4,978,567, the entire disclosure of which is herebyincorporated herein by reference.

[0084] Although this invention has been disclosed in the context ofcertain preferred embodiments and examples, it will be understood bythose skilled in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications thereof. Thus, itis intended that the scope of the present invention herein disclosedshould not be limited by the particular disclosed embodiments describedabove, but should be determined only by a fair reading of the claimsthat follow.

What is claimed is:
 1. A susceptor for supporting a wafer within areaction chamber, comprising: an upper support surface configured tosupport a wafer; a plurality of gas passages within the susceptor, thegas passages having inlet ends configured to receive a gas flow from asource of gas, and having outlet ends opening at said upper supportsurface; a gas supply system configured to supply a generally upwardflow of gas through said gas passages, said passages configured so thatsuch a gas flow would apply an upwardly directed force to a wafer abovesaid upper support surface, said gas supply system configured to supplya flow rate of gas sufficient to slow the rate of descent of a fallingwafer above said upper support surface to a rate of descent no greaterthan one half of the rate at which the wafer would descend under gravityalone.
 2. The susceptor of claim 1, wherein said gas supply system isconfigured to supply a flow rate of gas sufficient to slow the rate ofdescent of a falling wafer above said upper support surface to a rate ofdescent no greater than one quarter of the rate at which the wafer woulddescend under gravity alone.
 3. The susceptor of claim 1, wherein saidgas supply system is configured to supply a flow rate of gas sufficientto slow the rate of descent of a falling wafer above said upper supportsurface to a rate of descent no greater than one quarter of the rate atwhich the wafer would descend under gravity alone.
 4. The susceptor ofclaim 1, wherein said gas supply system is configured to supply a flowrate of gas sufficient to lift a wafer resting upon said upper supportsurface.
 5. The susceptor of claim 1, said support surface including aplurality of grooves and protrusions, wherein tops of said protrusionsare configured to support a wafer, said grooves configured to permit adegree of gas flow underneath the wafer and upward around a peripheraledge of the wafer when the wafer is supported upon said tops of saidprotrusions.
 6. A substrate holder comprising: a susceptor including aplurality of gas passages and a support surface; and a gas supply systemconfigured to supply an upwardly directed flow of gas through said gaspassages, said gas supply system configured to supply a flow rate of gassufficient to slow the rate of descent of a 100 mm substrate that isabove and falling toward said support surface to a rate of descent nogreater than one half of the rate at which the 100 mm substrate woulddescend under gravity alone.
 7. The substrate holder of claim 6, whereinsaid gas supply system is configured to supply a flow rate of gassufficient to slow the rate of descent of a 300 mm substrate that isabove and falling toward said support surface to a rate of descent nogreater than one half of the rate at which the 300 mm substrate woulddescend under gravity alone.
 8. The substrate holder of claim 6, whereinsaid support surface is defined by tops of a plurality of protrusions,the protrusions being separated by a plurality of crossing grid grooves.9. The substrate holder of claim 6, wherein said susceptor is formed ofat least two independent sections.
 10. The substrate holder of claim 6,further comprising a spider assembly configured to support saidsusceptor, said spider assembly including a central hub and a pluralityof spider arms extending radially outward and upward from said centralhub, said central hub and said spider arms being hollow to permit saidgas to flow upward through said spider assembly into said susceptor. 11.The substrate holder of claim 10, wherein said spider assembly isconfigured to rotate about a vertical axis and to thereby rotate saidsusceptor.
 12. The substrate holder of claim 10, wherein said susceptorcomprises: a lower section having a bottom surface and a top surface,said bottom surface of said lower section having a plurality of spiderarm recesses adapted to receive upper ends of said spider arms of saidspider assembly, said top surface of said lower section having a set ofinterconnected grooves, said lower section having internal passagesextending from said spider arm recesses to said grooves to permit gasflowing upward through said spider arms to flow through said internalpassages into said grooves of said lower section; and an upper sectionhaving a lower recess configured to receive said lower section, saidupper section having a lower surface and a top surface, said pluralityof gas passages being within said upper section, each of said gaspassages having a lower end positioned above one of said grooves in saidtop surface of said lower section so that gas in said grooves of saidlower section can flow into said gas passages in said upper section,each of said gas passages having an upper end opening at said topsurface of said upper section so that gas within said gas passages canflow upward into the space above said susceptor.
 13. The substrateholder of claim 6, wherein at least some of said gas passages areoriented substantially diagonally such that gas flowing upward throughsaid at least some gas passages exits said at least some gas passages byflowing upward and radially outward.
 14. The substrate holder of claim6, wherein said susceptor is designed to hold a substrate having apredetermined size, some of said gas passages being substantiallyvertically oriented and positioned so as to be just radially outward ofa peripheral edge of a substrate of said predetermined size and centeredon said susceptor.
 15. The substrate holder of claim 6, wherein saidplurality of gas passages comprises at least twenty passages.
 16. Thesubstrate holder of claim 6, wherein said gas passages are positionallybalanced with respect to said support surface of said susceptor, suchthat an upward flow of said gas through said passages causessubstantially balanced application of force onto a substrate above saidsupport surface.
 17. A method of supporting a substrate, comprising:releasing a substrate above a support surface of a susceptor, such thatsaid substrate is permitted to descend toward said support surface bygravitational force; providing a cushioning flow of gas upwardly througha plurality of gas passages provided in said susceptor, said cushioninggas flow providing an upwardly directed force onto said substrate, theflow rate of said cushioning gas flow being sufficient to slow the rateof descent of said substrate to a rate no greater than one half of therate at which said substrate would descend under gravity alone; andpermitting said substrate to come into contact with said supportsurface.
 18. The method of claim 17, wherein said susceptor includes aplurality of outer gas passages positioned just radially outward of aperipheral edge of said substrate when said substrate is centered uponsaid support surface of said susceptor, said outer gas passages beingsubstantially vertical so that upward gas flow through said outer gaspassages flows substantially vertically upward above said susceptor,said method further comprising providing gas flow through said outer gaspassages to counteract sideward sliding of said substrate.
 19. Themethod of claim 18, wherein said gas flow is provided through said outergas passages while said substrate descends toward said support surfaceof said susceptor.
 20. The method of claim 18, wherein said gas flow isprovided through said outer gas passages while said substrate is incontact with said support surface of said susceptor.
 21. The method ofclaim 17, wherein said cushioning flow of gas exits each of said gaspassages at said support surface by flowing upward and radially outward.22. The method of claim 17, said support surface having a plurality ofgrooves extending radially outward beyond the peripheral edge of saidsubstrate, said method further comprising: providing a trickle flow ofgas through said plurality of gas passages after said substrate comesinto contact with said support surface and while processing saidsubstrate, the flow rate of said trickle gas flow being sufficient tocause said gas to flow under said substrate through said grooves andupward around a peripheral edge of said substrate without disruptingcontact between said substrate and said support surface.
 23. The methodof claim 17, further comprising: after said substrate is in contact withsaid support surface, providing a liftoff flow of gas through saidplurality of gas passages, said liftoff gas flow being sufficient tolift said substrate off of said support surface; and removing saidsubstrate from above said support surface.
 24. The method of claim 23,wherein said susceptor includes a plurality of outer gas passagespositioned just radially outward of a peripheral edge of said substratewhen said substrate is centered upon said support surface of saidsusceptor, said outer gas passages being substantially vertical so thatupward gas flow through said outer gas passages flows substantiallyvertically upward above said susceptor, said method further comprisingproviding gas flow through said outer gas passages to counteractsideward sliding of said substrate after said liftoff gas flow isprovided.
 25. The method of claim 17, wherein said susceptor isconfigured with a pocket to support a 300 mm semiconductor substrate.26. The method of claim 17, wherein said susceptor is configured with apocket to support a 200 mm semiconductor wafer.
 27. The method of claim17, wherein said susceptor is configured with a pocket to support a 150mm semiconductor wafer.
 28. The method of claim 17, wherein said rate ofdescent of said substrate is no greater than one third of the rate atwhich said substrate would descend under gravity alone.
 29. The methodof claim 17, wherein said rate of descent of said substrate is nogreater than one quarter of the rate at which said substrate woulddescend under gravity alone.
 30. The method of claim 17, wherein at themoment that said substrate comes into contact with said support surface,the temperature difference between said substrate and said supportsurface is no greater than 100° C.
 31. The method of claim 17, whereinsaid contact between said substrate and said support surface produces acurl of said substrate of no greater than 0.200 inches.
 32. The methodof claim 17, wherein said contact between said substrate and saidsupport surface produces a curl of said substrate of no greater than0.100 inches.
 33. The method of claim 17, wherein said contact betweensaid substrate and said support surface produces a curl of saidsubstrate of no greater than 0.050 inches.
 34. The method of claim 17,wherein said susceptor has a temperature within the range of 200-1000°C., and said substrate has a temperature within the range of 0-100° C.at the time of said releasing of said substrate.
 35. A method of loadinga substrate onto a susceptor, comprising: releasing a substrate above asupport surface of a susceptor, such that said substrate is permitted todescend toward said support surface by gravitational force; providing acushioning flow of gas that imparts an upwardly directed force onto saidsubstrate, the flow rate of said cushioning gas flow being sufficient toslow the rate of descent of said substrate to a rate no greater than onehalf of the rate at which said substrate would descend under gravityalone; and permitting said substrate to come into contact with saidsupport surface.
 36. A method of loading a wafer onto a susceptor,comprising: releasing a wafer above a susceptor, said wafer having aperipheral edge; and providing a plurality of substantially verticalupwardly directed gas jets radially exterior of said peripheral edge ofsaid wafer, said jets substantially inhibiting sideward motion of saidwafer as said wafer descends toward said susceptor.